Seem to have a workaround for this now - am using essentially the same GLSL code but swap it for OGL 1.2, 3.3 and OGLES 2.0 versions. The main change is the first line of the shader source code files which gives the version number. I will post links to my code once it's uploaded and stable.

I haven't managed to force OGLES 2.0 on the ExoPC but it seems largely irrelevent now. Hope this helps someone else!